Superconductivity from valence fluctuations

Electron-phonon coupling is, so far, the only realizable source of attractive potential for conventional (s-wave) superconductivity in condensed matter systems. In two prototypical heavy-fermion superconductors, CeCoIn₅, and CeCu₂Si₂, recent experiments have suggested the presence of conventional superconductivity in a small parameter space, where decades of studies suggested unconventional pairings. Interestingly, the materials have insufficient electron-phonon coupling strength to cause such conventional pairing. Here we present a theory of superconductivity from valence fluctuations between conduction and localized bands which are present in heavy-fermion materials. We find that when we account of multiple valence fluctuations, there is a solution for robust conventional superconductivity with typical s-wave pairing symmetry. The results are consistent with recent experimental results.